Controlled type magnetic bearing devices, which magnetically levitate a rotary body for supporting in a non-contact state, have been widely used in various fields in recent years since the loss accompanied by rotation can be remarkably reduced.
Generally a controlled type magnetic bearing device is provided with a group of controlled type magnetic axial bearings which support a rotary body in a non-contact state in an axial direction and two groups of controlled type magnetic radial bearings which support the rotary body in a non-contact state in a radial direction. The magnetic radial bearing is equipped with plural electromagnets (usually four pieces) which are arranged at equal intervals in a circumferential direction of the rotary body. Each of the electromagnets is provided with a magnetic pole having a salient pole, which projects inside in a radial direction and opposes an outer periphery of the rotary body, and a coil for feeding an exciting electric current is wound around the magnetic pole. The exciting current is combined with a constant bias current and a control current controlled dependent on the displacement of the rotary body. A magnetic flux combined with a constant bias magnetic flux by the bias current and a control magnetic flux by the control current is formed in each of the magnetic poles.
In such a controlled type magnetic radial bearing, there is a problem caused by eddy current loss produced inside a rotary body at a high-speed rotation. An eddy current is generated when a flux density varies in a circumferential direction of the rotary body and the magnitude of the eddy current is proportional to the variation of flux density.
There are hetero-polar type and homo-polar type magnetic radial bearings according to the structure of an electromagnet. In the hetero-polar type magnetic radial bearing, each of the electromagnets has magnetic poles at two positions in a circumferential direction of the rotary body and the two magnetic poles are excited in opposite polarities by supplying an exciting current to coils. Since a plurality of such electromagnets are arranged in the circumferential direction of the rotary body, the magnetic poles with reversed polarities are arranged adjacently to each other in the circumferential direction of the rotary body. Therefore, the flux density around the rotary body is largely varied along the circumferential direction of the rotary body, a large eddy current flows inside the rotary body to generate an eddy current loss and increase the rotation loss.
On the other hand, in the homo-polar type magnetic radial bearing, each of the electromagnets has magnetic poles in two positions in the axial direction of a rotary body and two magnetic poles are excited with reversed polarities by supplying an exciting current to coils. By supplying the exciting currents so that the magnetic poles on the same side in the axial direction of the rotary body may become the same polarity, only the magnetic poles with the same polarity are arranged on the same side in the circumferential direction of the rotary body. Therefore, the change of the flux density becomes small along the circumferential direction of the rotary body. However, since there is still a difference of flux density between a portion with a magnetic pole and a portion with no magnetic pole, the change of flux density is generated between the magnetic poles arranged adjacently to each other in the circumferential direction of the rotary body. Therefore, approximately half the eddy current in the case of the hetero-polar type described above flows to generate approximately half the rotation loss of the hetero-polar type.
The eddy current described above can be decreased to a level having substantially no problem for an ordinary device by using laminated steel plates for the rotor part of a rotary body opposing the electromagnet. However, there have been increasing applications in recent years in which the loss of eddy current and the rotation loss due to the eddy current practically cause problems even though laminated steel plates are used for the rotor part of a rotary body in a device such as an energy-storage flywheel and an ultra high-speed rotary body.
Therefore, it is an object of the present invention to provide a controlled type magnetic radial bearing which is capable of suppressing generation of an eddy current to reduce the rotation loss in order to solve the above-mentioned problem.